The Collaborative International Dictionary of English v.0.48:

Arch \Arch\ ([aum]rch), n. [F. arche, fr. LL. arca, for arcus.
   See Arc.]
   1. (Geom.) Any part of a curved line.
      [1913 Webster]

   2. (Arch.)
      (a) Usually a curved member made up of separate
          wedge-shaped solids, with the joints between them
          disposed in the direction of the radii of the curve;
          used to support the wall or other weight above an
          opening. In this sense arches are segmental, round (i.
          e., semicircular), or pointed.
      (b) A flat arch is a member constructed of stones cut into
          wedges or other shapes so as to support each other
          without rising in a curve.
          [1913 Webster]

   Note: Scientifically considered, the arch is a means of
         spanning an opening by resolving vertical pressure into
         horizontal or diagonal thrust.
         [1913 Webster]

   3. Any place covered by an arch; an archway; as, to pass into
      the arch of a bridge.
      [1913 Webster]

   4. Any curvature in the form of an arch; as, the arch of the
      aorta. "Colors of the showery arch." --Milton.
      [1913 Webster]

   Triumphal arch, a monumental structure resembling an arched
      gateway, with one or more passages, erected to commemorate
      a triumph.
      [1913 Webster]

WordNet (r) 3.0 (2006):

triumphal arch
    n 1: a monumental archway; usually they are built to commemorate
         some notable victory